This disclosure relates to circuits and materials useful in the manufacture thereof, and in particular, to circuit materials comprising a thermoset liquid crystalline polymer.
As used herein, a circuit material is an article used in the manufacture of circuits, and includes circuit laminates, bond plies, cover films, and resin coated conductive layers. Circuit laminates comprise one or two conductive layers fixedly attached to a dielectric substrate, which is formed from a dielectric material. The dielectric material in a circuit laminate, bond ply, or resin coated conductive layer can comprise a thermosetting or thermoplastic polymer. The dielectric substrate in a circuit laminate generally does not soften or flow during manufacture or use of the circuit, while the dielectric material in a bond ply and resin coated conductive layer usually softens and flows during manufacture of the circuit. Dielectric materials are typically divided into two classes, flexible and rigid. Flexible dielectrics generally tend to be thinner and more bendable than the so-called rigid dielectric layers, which typically comprise a fibrous web or other forms of reinforcement, such as short or long fibers and/or fillers.
Patterning a conductive layer of a circuit laminate, for example by etching, provides a circuit layer. Multi-layer circuits comprise a plurality of conductive layers, at least one of which contains a conductive wiring pattern. Typically, multi-layer circuits are formed by laminating one or more circuit layers together using bond plies, in proper alignment, using heat and/or pressure. The bond plies are used to provide adhesion between circuits and/or between a circuit and a conductive layer that is subsequently patterned. In multi-layer structures, after lamination, known hole forming and plating technologies may be used to produce useful electrical pathways between conductive layers. A resin coated conductive layer may be bonded directly to the outer layer of a circuit. This resin coated conductive layer is sometimes referred to as a cap layer.
Electronic devices that operate at high frequencies (1 gigahertz (GHz) or higher) require use of circuit substrates with low dielectric constants and low dissipation factors. In addition, the requirements for circuit materials and circuits in high density, high performance applications require circuit materials having low dielectric constants (Dk) for low propagation delay, lower cross talk and higher clock rates, low dissipation factor (Df) for low attenuation, better signal integrity, and lower power consumption in portables. In addition, the circuit should be non-flammable, preferably achieving a rating of V-1 or better in the Underwriter's Laboratory UL-94 flammability test without the addition of halogenated flame retardants, as some of these face potential ban by the European Legislation. In addition, the circuit materials preferably have low coefficients of thermal expansion (CTE) in all directions to provide good dimensional stability and enhanced reliability, e.g., plated through-hole reliability.
Accordingly, there continues to be a need in the art for circuit materials and circuits suitable for use in high density, high performance applications. A number of crosslinked liquid crystalline polymers are known, but suffer from various drawbacks. For example, U.S. Pat. No. 5,439,541 to Economy discloses the use of cross-linked co-polyesters in circuit applications, wherein the melting point and cure temperature of the co-polyesters are relatively close in value. This proximity in melting point and cure temperature renders it difficult to produce the co-polyester in sheet form and to make partially cured laminates, which are useful for making homogeneous bond ply systems.
U.S. Pat. Nos. 5,114,612 and 5,475,133, both to Benicewicz et al., disclose the manufacture of liquid crystalline thermosets using, in the Examples, monomers that are either liquids or very brittle powders, thereby making it difficult to form sheets. These monomers also have very high exothermic curing reactions which are difficult to control, and which result in undesirable porous laminates. Additionally, as these monomers are not film forming, it is difficult to make B-staged prepregs with them.
European Patent, EP 0 697 278 B1 discloses the formation of rigid laminates from thermoplastic liquid crystalline polymers. The resulting laminates suffer from a high coefficient of thermal expansion in the Z-axis, and a low modulus and cohesive strength at high temperatures, which yields a low quality circuit board and poor bond strength at high temperatures, thereby making it difficult for the circuit board to withstand soldering and other high temperature assembly applications.
Accordingly, there continues to be a need in the art for circuit materials and circuits that meet the needs of high density, high performance applications.